1. Field of the Invention
This invention is related to the field of Optical Coherence Tomography (OCT) imaging systems and in particular to a light source with an optical spectrum exhibiting a discrete set of narrow peaks having high optical power, to be used with OCT imaging systems.
2. Description of the Related Arts
Optical Coherence Tomography (OCT) is a fast and accurate optical imaging technique frequently used in producing high-resolution diagnostic images for a variety of diagnostics and clinical applications. Currently, most common commercial application of OCT is primarily in the field of ophthalmology. Emerging applications include but are not limited to, field of cardiology, dentistry, cancer diagnosis, glucose monitoring, and dermatology to name a few. As the technology matures and becomes cost effective, applications of OCT may be expanded to other fields in future.
Primarily, OCT is an interferometric technique in which, the light from a broadband or a tunable source is split into a reference arm and a sensing arm of an interferometer. Light from the two arms are made to interfere and the interferometric signal detected by a detector is processed in time domain or frequency domain, to obtain an image of a sample-for example, a retina or a fundus in ophthalmological diagnostics. An important condition to detect an interference signal is that the optical path difference between the two arms of the interferometer is shorter than the coherence length of the light source.
In principle, OCT can be configured to operate in a “time domain” (TD-OCT) imaging mode or in a Fourier-domain (FD-OCT) imaging mode. In a TD-OCT system, length of a reference arm is linearly scanned to match depth locations in a test sample placed in a scanning arm to obtain a depth profile features of the sample. While the method is accurate, limitations of a conventional TD-OCT system is well documented in the U.S. Pat. No. 7,697,145 issued to Izatt on Apr. 13, 2010 which is being incorporated by reference in its entirety. Some of the limitations are, complexity and relatively low speed of mechanical scanning devices, single detector that serially obtains signal to build a sample image pixel by pixel, and low source output power resulting in a need for signal integration at the detector that results in low imaging speed.
On the other hand, the FD-OCT system offers about 100 times more sensitivity and about 50-100 times faster image acquisition speed. Two equivalent FD-OCT configurations are currently being considered particularly for medical applications—a Swept Source (SS)-OCT configuration (SS-system hereinafter) and a Spectral Domain (SD)-OCT configuration (SD-system hereinafter). With the same average source power, the performances of the SS-system and SD-system are identical regarding data acquisition rate and return loss. However, there are differences in systems cost, complexity, and speed capability.
In a SS-system, the wavelength of a single broadband tunable source is sequentially scanned to produce a depth profile of the sample in the frequency domain. A single detector sequentially detects the light and produces an interference signal. The interference signal is the cross-product of the light reflected from the reference and sensing arms and it is produced by the detector which is a square law device. The speed of imaging is limited by the scanning speed of the tunable laser. In addition, the swept source is complex and bulky. Only complicated experimental systems have been shown to reach the 10-15 KHz claimed speed. Most commercial SS-systems operate at a slower rate. In a swept source OCT system disclosed in the U.S. Pat. No. 7,697,145 issued to Izatt on Apr. 13, 2010, a tunable narrow band source whose frequency is swept with time, is used to image a sample using phase sensitive multi-channel detection system to improve the imaging speed.
In the U.S. Pat. No. 7,391,520 issued to Zhou et al. on Jun. 24, 2008, an alternative FD-OCT system is disclosed where a multi-wavelength laser is used as a light source. The advantage of this system is that the sweeping range of each individual laser is substantially reduced as compared to a single wavelength laser to cover the same spectral range. A multi-channel receiver detects the interferometric signal thereby increasing the speed of each axial scan. The disclosure of the U.S. Pat. No. 7,391,520 is hereby incorporated by reference in its entirety.
The alternative to the SS-system is a SD-system that has two potential advantages in terms of speed as has been disclosed in several U.S. Pat. No. 7,557,931 issued to Toida on Jul. 7, 2009, U.S. Pat. No. 7,826,059 issued to Roth et al. on Nov. 10, 2010, U.S. Pat. No. 7,848,791 issued to Schmitt et al. on Dec. 7, 2010 and U.S. Pat. No. 7,872,761 issued to Pedro et al. on Jan. 18, 2011, each one being incorporated by reference in their entirety.
A common feature of systems described in these patents is that a broadband light source such as a Super Luminescent Diode (SLD) or an Amplified Spontaneous Emission (ASE) light source is used. Since the interference signal in a SD-system is generated using light from a broadband source a dispersive device such as a grating is necessary to separate the interfering light (the combined reference and sensing lights) into its spectral components. Each spectral component can then be detected simultaneously using a photo-detector array to produce the interference signal. The detector array can be read out at a much faster rate and signal from the detector can be processed simultaneously or sequentially, to produce an image at a faster rate than in a SS-system.
However, since currently available SLD and ASE sources exhibit low power density, power per pixel received at the detector is comparatively low which may result in lower sensitivity. In order to achieve higher sensitivity, a longer signal integration time is required at the detector array thereby, limiting the speed of imaging. Imaging speed for the SD-system can be improved by increasing the signal at the detector elements so as to reduce integration time at the detector. Therefore SD-system although more affordable, may require a trade-off between sensitivity and speed.
A preferred source for the SD-system would be the one in which the light source has more input power such that the interfering signal reaches the detector with more power. An alternative light source having a discrete spectrum of high-power spikes may be a more desirable option in improving the OSNR (Optical Signal to Noise Ratio) at the detector and allow higher imaging speed than SS-OCT. More importantly, if the discrete spectrum of the source matches the spatial distribution of the detector array elements, high accuracy images may be obtained in a shorter time.
In the United States Patent Application Publication No. 2009/0262359 by Bajraszewski et al. on Oct. 22, 2009, a comb generator is disclosed where light from a source traverses through an adjustable Fabry-Perot device to generate multiple peaks. One mirror of the adjustable Fabry-Perot device is mounted on a positioning device to control positions of the spectral components of the optical frequency comb such that elements of a detector array may be aligned with the peaks of the optical frequency comb.